Boost C++ Libraries

The monadic interface of optional allows the application of functions to optional values without resorting to the usage of explicit if-statements.

Function map takes a function mapping type T onto type U and maps an optional<T> onto an optional<U> using the provided function.

int length(const string& s){ return s.size(); };

optional<string> null{}, thin{""}, word{"word"};
assert (null.map(length) == none);
assert (thin.map(length) == 0);
assert (word.map(length) == 4);

Function flat_map is similar, but it requires the function to return an optional<V> for some type V. This optional<V> becomes the return type of flat_map.

optional<char> first_char(const string& s) {
  if (s.empty()) return none;
  else           return s[0];
};

optional<string> null{}, thin{""}, word{"word"};
assert (null.flat_map(first_char) == none);
assert (thin.flat_map(first_char) == none);
assert (word.flat_map(first_char) == 'w');

These functions can be combined in one expression reflecting a chain of computations:

auto get_contents(path p) -> optional<string>;
auto trim(string) -> string;
auto length(string) -> int;

auto trimmed_size_of(optional<path> p) -> int
{
  return p.flat_map(get_contents)
          .map(trim)
          .map(length)
          .value_or(0);
}